The Metal-Dependent Regulators FurA and FurB from Mycobacterium Tuberculosis.

Lucarelli D, Vasil ML, Meyer-Klaucke W, Pohl E - Int J Mol Sci (2008)

Bottom Line:
The ferric uptake regulators (Fur) form a large family of bacterial metal-activated DNA-binding proteins that control a diverse set of genes at the transcriptional level.Although both belong to the same family, they share only approximately 25% sequence identity and as a consequence, they differ significantly in some of their key biological functions.Recent biochemical, crystallographic and spectroscopic data have now shed light on the activation and metal discrimination mechanisms in this protein family.

ABSTRACTThe ferric uptake regulators (Fur) form a large family of bacterial metal-activated DNA-binding proteins that control a diverse set of genes at the transcriptional level. Mycobacterium tuberculosis, the causative agent of tuberculosis, expresses two members of the Fur family, designated FurA and FurB. Although both belong to the same family, they share only approximately 25% sequence identity and as a consequence, they differ significantly in some of their key biological functions. FurA appears to be a specialized iron-dependent regulator that controls the katG gene, which encodes for a catalase-peroxidase involved in the response of M. tuberculosis to oxidative stress. KatG is also the key mycobacterial enzyme responsible for the activation of the first-line tuberculosis drug Isoniazid. FurB in contrast requires Zn(2+) rather than Fe(2+), to bind to its target sequence in regulated genes, which include those involved in Zn(2+)-homeostasis. Recent biochemical, crystallographic and spectroscopic data have now shed light on the activation and metal discrimination mechanisms in this protein family.

f1-ijms-9-1548: Schematic representation of the regulatory functions of M. tuberculosis FurA and FurB, respectively. Full arrows refer to proven repression, dashed arrows refer to proposed activation pathways. Note, that the structural Zn2+-site present in both FurA and FurB has been omitted for clarity.

Mentions:
The furA gene is located immediately upstream of the katG gene encoding a catalase-peroxidase. KatG is involved in the oxidative stress response and a significant virulence factor of M. tuberculosis [32]. FurA is co-expressed along with KatG and it auto-represses its expression by binding to a unique sequence upstream of the furA gene [33]. Although there are indications that this protein is also involved in the regulation of other genes [32], the biological role of FurA, in contrast to the role of most members of the family appears to be more specialized (Figure 1). FurA could represent the metal-dependent peroxide sensor similar to the peroxide regulon repressor PerR that has been extensively characterized in B. subtilis [34]. Both proteins share a sequence identity of approximately 28% (Figure 2). PerRbs adopts a similar fold to Furpa and contains one structural Zn2+- and one regulatory Fe2+/Mn2+-binding site [35]. Once activated and bound to its target DNA-sequence the regulator senses oxidative stress by iron-catalyzed histidine oxidation that leads to the loss of DNA-binding activity [36]. FurA is of particular biomedical importance as it controls a protein central to the contemporary TB therapy. One of the more efficacious therapeutics for this disease is Isoniazid (isonicotinic acid hydrazide = INH), which is able to traverse the complex lipid membrane of M. tuberculosis, entirely by passive diffusion. While unmodified INH is not toxic to the pathogen it becomes activated by the mycobacterial catalase KatG, which modifies INH into a range of reactive intermediates including NAD+ and NADP+ adducts. These then act as potent inhibitors of the NADPH-dependent enoyl acyl carrier protein reductase (InhA) of the fatty acid synthase type II [37, 38]. InhA is an essential enzyme in the mycolic acid synthesis [39]. INH-resistant M. tuberculosis strains isolated from patients predominantly have mutations in the katG gene leading to a catalase-peroxidase with reduced or abolished catalytic activity [38]. In addition, mutations were also found in the inhA gene as well as various other genes including some in the furA gene [38, 40]. FurA has attracted increased interest as a potentially novel drug target for the development of inhibitors that could enhance KatG levels possibly boosting INH potency.

f1-ijms-9-1548: Schematic representation of the regulatory functions of M. tuberculosis FurA and FurB, respectively. Full arrows refer to proven repression, dashed arrows refer to proposed activation pathways. Note, that the structural Zn2+-site present in both FurA and FurB has been omitted for clarity.

Mentions:
The furA gene is located immediately upstream of the katG gene encoding a catalase-peroxidase. KatG is involved in the oxidative stress response and a significant virulence factor of M. tuberculosis [32]. FurA is co-expressed along with KatG and it auto-represses its expression by binding to a unique sequence upstream of the furA gene [33]. Although there are indications that this protein is also involved in the regulation of other genes [32], the biological role of FurA, in contrast to the role of most members of the family appears to be more specialized (Figure 1). FurA could represent the metal-dependent peroxide sensor similar to the peroxide regulon repressor PerR that has been extensively characterized in B. subtilis [34]. Both proteins share a sequence identity of approximately 28% (Figure 2). PerRbs adopts a similar fold to Furpa and contains one structural Zn2+- and one regulatory Fe2+/Mn2+-binding site [35]. Once activated and bound to its target DNA-sequence the regulator senses oxidative stress by iron-catalyzed histidine oxidation that leads to the loss of DNA-binding activity [36]. FurA is of particular biomedical importance as it controls a protein central to the contemporary TB therapy. One of the more efficacious therapeutics for this disease is Isoniazid (isonicotinic acid hydrazide = INH), which is able to traverse the complex lipid membrane of M. tuberculosis, entirely by passive diffusion. While unmodified INH is not toxic to the pathogen it becomes activated by the mycobacterial catalase KatG, which modifies INH into a range of reactive intermediates including NAD+ and NADP+ adducts. These then act as potent inhibitors of the NADPH-dependent enoyl acyl carrier protein reductase (InhA) of the fatty acid synthase type II [37, 38]. InhA is an essential enzyme in the mycolic acid synthesis [39]. INH-resistant M. tuberculosis strains isolated from patients predominantly have mutations in the katG gene leading to a catalase-peroxidase with reduced or abolished catalytic activity [38]. In addition, mutations were also found in the inhA gene as well as various other genes including some in the furA gene [38, 40]. FurA has attracted increased interest as a potentially novel drug target for the development of inhibitors that could enhance KatG levels possibly boosting INH potency.

Bottom Line:
The ferric uptake regulators (Fur) form a large family of bacterial metal-activated DNA-binding proteins that control a diverse set of genes at the transcriptional level.Although both belong to the same family, they share only approximately 25% sequence identity and as a consequence, they differ significantly in some of their key biological functions.Recent biochemical, crystallographic and spectroscopic data have now shed light on the activation and metal discrimination mechanisms in this protein family.

ABSTRACTThe ferric uptake regulators (Fur) form a large family of bacterial metal-activated DNA-binding proteins that control a diverse set of genes at the transcriptional level. Mycobacterium tuberculosis, the causative agent of tuberculosis, expresses two members of the Fur family, designated FurA and FurB. Although both belong to the same family, they share only approximately 25% sequence identity and as a consequence, they differ significantly in some of their key biological functions. FurA appears to be a specialized iron-dependent regulator that controls the katG gene, which encodes for a catalase-peroxidase involved in the response of M. tuberculosis to oxidative stress. KatG is also the key mycobacterial enzyme responsible for the activation of the first-line tuberculosis drug Isoniazid. FurB in contrast requires Zn(2+) rather than Fe(2+), to bind to its target sequence in regulated genes, which include those involved in Zn(2+)-homeostasis. Recent biochemical, crystallographic and spectroscopic data have now shed light on the activation and metal discrimination mechanisms in this protein family.